1. Field of the Invention
The present invention relates to a process for preparing a porous sheet material. Particularly, it relates to a process which is capable of providing a porous sheet material superior in its properties such as mechanical strength and steam permeability at a high level of productivity.
2. Description of the Prior Art
A number of processes have been known for the preparation of porous sheet materials, as substitutes for natural leathers, from polyurethanes. Such processes may be generally classified into wet processes and dry processes. Each type of processes has its own pros and cons. The dry processes are superior from the viewpoint of the productivity. As such dry processes, there may be mentioned, for instance, processes disclosed in Japanese Examined Patent Publication No. 18249/1980 and Japanese Unexamined Patent Publication No. 41063/1976. However, in these conventional processes, a relatively great amount of a hydrophilic polyurethane is used for the formation of the polyurethane layer, and accordingly, the mechanical properties and physicochemical properties of the sheet material thereby obtainable tend to be inadequate. Further, in the conventional processes, a cumbersome and time-consuming drying step constitutes a decisive drawback. It is naturally conceivable that the amount of the hydrophilic polyurethane be reduced to overcome this drawback. However, if the amount is so reduced, there will be other drawbacks (for instance, instability of the treating liquid).
The present inventors have found that it is possible to overcome the drawbacks of the conventional processes by employing a hydrophobic polyurethane dispersion prepared by a specific method, as the major component of the coating material, and using a small amount of a hydrophilic polyurethane containing polyoxyethylene groups in a specific proportion, as the emulsifier. The present invention is based on this discovery.